The present invention relates to a system for compensation for weather-based attenuation in a satellite link and, more particularly, but not exclusively to such compensation wherein a range of modulation levels can be selected. The present methods can be used in either one direction, that is forward or return links, or in both directions (forward and return links).
Weather can cause attenuation to the signal on a satellite communication link. Furthermore the ground to satellite leg may experience different weather conditions from the satellite to ground leg. Furthermore, in a broadcast system, different satellite to ground legs may experience different weather conditions, so that the overall attenuation in the link may not only change rapidly but may differ between different receiving stations at the same instant.
A number of solutions have been used in the past. One popular solution involves designing the satellite communication system at the outset for the worst case weather conditions. Such a solution is particularly wasteful of power although it rarely fails. Another solution involves using climatology to estimate weather parameters of concern, and then setting the transmission power for the estimated conditions. Further solutions use empirical models based on climatological data and long baseline observations of signal strength to model RF attenuation and compensate accordingly.
A recent proposal involves operating the link based on expected daily weather conditions for the specific geographical region in which the link operates. However even in this case transmission power is wasted since the system operates on the basis of the worst case within the time and geographical frame of the estimate.
It is known to provide automatic uplink power control (AUPC), that is, adjusting the output power on the uplink, with the general aim of maintaining a constant signal to noise ratio at the remote end. This is however inexact as the control over the downlink is indirect.
Adaptive coding and modulation (ACM) is known to keep the SNR of the channel constant in the face of changing noise levels. The modulation pattern is changed between a high capacity modulation at low noise and a low capacity but highly robust modulation when the noise increases.
However attempts to dynamically change the modulation based on the requirements of the system as a whole lead to problems with individual links. Likewise attempts to dynamically change individual links lead to imbalances in the system as a whole.
The following documents are representative of the state of the art:
Thomas J. Saam, “Uplink Power Control Technique for VSAT Networks”, in Proceedings of Souteastcon 89, pp. 96-101, April 1989.
Thomas J. Saam, “Uplink power control mechanism for maintaining constant output power from satellite transponder”, U.S. Pat. No. 4,941,199, Filed Apr. 6, 1989.
Lawrence W. Krebs et al., “Methods and Apparatus For Mitigating Rain Fading Over Satcom Links Via Information Throughput Adaptation, U.S. Pat. No. 7,174,179, Filed Feb. 6, 2007.
ETSI EN 302 307 V1.1.1 (2004-01):“Digital Video Broadcasting (DVB) Second generation framing structure, channel coding and modulation systems for Broadcasting, Interactive Services, News Gathering and other broadband satellite applications”.
Alberto Morello, Vittoria Mignone, “DVB-S2: The Second Generation Standard for Satellite Broad-band Services”, Proceedings of the IEEE, vol. 94, no. 1, pp. 210-227, January 2006
G. Maral, M. Bousquet, Satellite Communications Systems, Third Edition, John Wiley & Sons, Ltd., 1999.